A recombinant murine retrovirus (MRSV) containing the src gene of avian Rous sarcoma virus was shown to induce hematopoietic colonies in a bone marrow colony-forming assay. Cells from the colonies induced by MRSV could be established as continuous transformed lines, all of which expressed high levels of pp60src, and appeared to be at an early stage in lymphoid differentiation. These findings provide the first demonstration that the src gene is capable of transforming cells of hematopoietic origin. BALB- and Harvey-murine sarcoma viruses (MSVs) were shown to alter the growth and differentiation of cells within the myeloid series in a manner that commonly leads to self-limited proliferation but infrequently induces neoplastic conversion, as well. Abelson-MuLV was shown to induce mast cell transformation in vitro when interleukin-3 was initially utilized to enhance the proliferation of the mast cell phenotype. Abelson-MuLV was the only retrovirus analyzed that conferred factor independence and leukemogenicity to the mast cell cultures.